arXiv subjects

A. C. Fabian

Publications and source records attributed to A. C. Fabian.

At least 505 records · Page 28Linked to original sources

arXiv2007

XMM-Newton observations of the Lockman Hole: X-ray source catalogue and number counts

The Lockman Hole field represents the sky area of lowest Galactic line-of-sight column density N_H=5.7X10^19 cm^-2. It was observed by the XMM-Newton X-ray observatory in 18 pointings for a total of 1.16 Msec (raw EPIC pn observing time) constituting the deepest XMM-Newton exposure so far. We present a catalogue of the X-ray sources detected in the central 0.196 deg^2 of the field and discuss the derived number counts and X-ray colours. In the 0.5--2.0 keV band, a sensitivity limit (defined as the faintest detectable source)of 1.9X10^-16 erg cm^-2 s^-1 was reached. The 2.0--10.0 keV band and 5.0--10.0 keV band sensitivity limits were 9X10^-16 erg cm^-2 s^-1 and 1.8X10^-15 erg cm^-2 s^-1, respectively.A total of 409 sources above a detection likelihood of 10 (3.9 sigma) were found within a radius of 15' off the field centre, of which 340, 266, and 98 sources were detected in the soft, hard, and very hard bands, respectively. The number counts in each energy band are in close agreement with results from previous surveys and with the synthesis models of the X-ray background. A 6% of Compton-thick source candidates have been selected from the X-ray colour-colour diagram. This fraction is consistent with the most recent predictions of X-ray background population synthesis models at our flux limits. We also estimated, for the first time, the logN-logS relation for Compton-thick AGN.

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arXiv2007

XMM-Newton and Broad Iron Lines

Iron line emission is common in the X-ray spectra of accreting black holes. When the line emission is broad or variable then it is likely to originate from close to the black hole. X-ray irradiation of the accretion flow by the power-law X-ray continuum produces the X-ray 'reflection' spectrum which includes the iron line. The shape and variability of the iron lines and reflection can be used as a diagnostic of the radius, velocity and nature of the flow. The inner radius of the dense flow corresponds to the innermost stable circular orbit and thus can be used to determine the spin of the black hole. Studies of broad iron lines and reflection spectra offer much promise for understanding how the inner parts of accretion flows (and outflows) around black holes operate. There remains great potential for XMM-Newton to continue to make significant progress in this work. The need for high quality spectra and thus for long exposure times is paramount.

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arXiv2007

The XMM-Newton Serendipitous Survey V. Optical identification of the XMM-Newton Medium sensitivity Survey (XMS)

We present the XMM-Newton Medium sensitivity Survey (XMS), including a total of 318 X-ray sources found among the serendipitous content of 25 XMM-Newton target fields. The XMS comprises four largely overlapping source samples selected at soft (0.5-2 keV), intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV) bands, the first three of them being flux-limited. We report on the optical identification of the XMS samples, complete to 85-95%. At the intermediate flux levels sampled by the XMS we find that the X-ray sky is largely dominated by Active Galactic Nuclei. The fraction of stars in soft X-ray selected samples is below 10%, and only a few per cent for hard selected samples. We find that the fraction of optically obscured objects in the AGN population stays constant at around 15-20% for soft and intermediate band selected X-ray sources, over 2 decades of flux. The fraction of obscured objects amongst the AGN population is larger (~35-45%) in the hard or ultra-hard selected samples, and constant across a similarly wide flux range. The distribution in X-ray-to-optical flux ratio is a strong function of the selection band, with a larger fraction of sources with high values in hard selected samples. Sources with X-ray-to-optical flux ratios in excess of 10 are dominated by obscured AGN, but with a significant contribution from unobscured AGN.

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arXiv2007

The Suzaku Observation of NGC 3516: Complex Absorption and the Broad and Narrow Fe K Lines

We present results from a 150 ksec Suzaku observation of the Seyfert 1.5 NGC 3516 in October 2005. The source was in a relatively highly absorbed state. Our best-fit model is consistent with the presence of a low-ionization absorber which has a column density near 5 * 10^{22} cm^{-2} and covers most of the X-ray continuum source (covering fraction 96-100%). A high-ionization absorbing component, which yields a narrow absorption feature consistent with Fe K XXVI, is confirmed. A relativistically broadened Fe K alpha line is required in all fits, even after the complex absorption is taken into account; an additional partial-covering component is an inadequate substitute for the continuum curvature associated with the broad Fe line. A narrow Fe K alpha emission line has a velocity width consistent with the Broad Line Region. The low-ionization absorber may be responsible for producing the narrow Fe K alpha line, though a contribution from additional material out of the line of sight is possible. We include in our model soft band emission lines from He- and H-like ions of N, O, Ne and Mg, consistent with photo-ionization, though a small contribution from collisionally-ionized emission is possible.

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arXiv2007

Constraints on light bending and reflection from the hard X-ray background

Light bending due to strong gravity has recently been invoked to explain variability and flux correlations between different bands in some accreting black holes. A characteristic feature of light bending is reflection-dominated spectra, especially if photon sources lie in the deepest parts of the gravitational potential within a few gravitational radii of the event horizon. We use the spectrum of the hard X-ray background in order to constrain the prevalence of such reflection-dominated sources. We first emphasize the need for reflection and explore the broad-band properties of realistic spectra that incorporate light bending. We then use these spectra, in conjunction with the observed 2-10 keV AGN distribution, evolutionary and obscuration functions in order to predict the hard X-ray background spectrum over 3-100 keV, and provide limits on the fraction of reflection-dominated objects, dependent on the height of the photon sources. Our results allow for a cosmologically-significant fraction of sources that incorporate strong light bending. The luminosity function based on intrinsic flare luminosities is derived and implications discussed. We discuss prospects for future hard X-ray missions such as NeXT and Simbol-X that can image such sources as well as confirm the precise spectral shape of the background near its peak, important for constraining the universal relevance of light bending.

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arXiv2007

The high energy emission of GRO J1655-40 as revealed with INTEGRAL spectroscopy of the 2005 outburst

We present broadband (3-500 keV) INTEGRAL X-ray spectra and X-ray/optical light curves of the luminous black hole X-ray transient and relativistic jet source GRO J1655-40. Our analysis covers four Target of Opportunity observations of the outburst that started in February 2005. We find that the high energy emission of GRO J1655-40 can be modelled well with an unbroken power-law (with photon indices of 1.72+-0.03,2.21+-0.04 for the first and the second observations, respectively). These correspond to hard and thermal dominant states, respectively. In contrast to many other black hole spectra, high energy complexity in the form of a break or cut-off is not required for the hard state, contrary to previous expectations for this state. We show for the first time that Comptonization by non-thermal electrons is the dominant process for the high energy emission in the hard state. We discuss our results in terms of models for broad-band emission and accretion flows in stellar-mass black holes.

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arXiv2007

X-ray active galactic nuclei in the core of the Perseus cluster

We present a study of the X-ray emission from the nuclei of galaxies observed in the core of the Perseus cluster in a deep exposure with Chandra. Point sources are found coincident with the nuclei of 13 early-type galaxies, as well as the central galaxy NGC1275. This corresponds to all galaxies brighter than M_B = -18 in the Chandra field. All of these sources have a steep power-law spectral component and four have an additional thermal component. The unabsorbed power-law luminosities in the 0.5-7.0 keV band range from 8.10^38-5.10^40 erg/s. We find no simple correlations between the K band luminosity, or the FUV and NUV AB magnitudes of these galaxies and their X-ray properties. We have estimated the black hole masses of the nuclei using the K band M_BH-L_K relation and again find no correlation between black hole mass and the X-ray luminosity. Bondi accretion onto the black holes in the galaxies with mini-haloes should make them much more luminous than observed.

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arXiv2007

X-ray reflection in accreting stellar-mass black hole systems

The X-ray spectra of accreting stellar-mass black hole systems exhibit spectral features due to reflection, especially broad iron K alpha emission lines. We investigate the reflection by the accretion disc that can be expected in the high/soft state of such a system. First, we perform a self-consistent calculation of the reflection that results from illumination of a hot, inner portion of the disc with its atmosphere in hydrostatic equilibrium. Then we present reflection spectra for a range of illumination strengths and disc temperatures under the assumption of a constant-density atmosphere. Reflection by a hot accretion disc differs in important ways from that of a much cooler disc, such as that expected in an active galactic nucleus.

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arXiv2007

Piecing Together the X-ray Background: Bolometric Corrections for Active Galactic Nuclei

(Abridged) The X-ray background can be used to constrain the accretion history of Supermassive Black Holes (SMBHs) in Active Galactic Nuclei (AGN). A knowledge of the hard X-ray bolometric correction, κ_{2-10keV} is a vital input into these studies. Variations in the disk emission in the UV have not previously been taken into account in calculating κ_{2-10keV}; we show that such variations are important by constructing optical--to--X-ray SEDs for 54 AGN. In particular, we use FUSE UV and X-ray data from the literature to constrain the disk emission as well as possible. Previous work has suggested a dependence of κ_{2-10keV} on AGN luminosity, but we find significant spread in κ_{2-10keV} with no simple dependence on luminosity. Populations such as Narrow-Line Seyfert 1 nuclei (NLS1s), Radio Loud and X-ray Weak AGN may have values of κ_{2-10keV} differing systematically from the rest of the AGN population. Other sources of uncertainty include intrinsic extinction in the optical--UV, X-ray and UV variability and uncertainties in SMBH mass estimates. Our results suggest a more well-defined relationship between κ_{2-10keV} and Eddington ratio in AGN, with a transitional region at an Eddington ratio of ~0.1, below which the bolometric correction is typically 15 - 25, and above which it is typically 40 - 70. We consider the potential implied parallels with the low/hard and high/soft states in Galactic Black Hole (GBH) accretion, and present bolometric corrections for the GBH binary GX 339-4 for comparison. Our findings reinforce previous studies proposing a multi-state description of AGN accretion analogous to that for GBH binaries. Future calculations of the SMBH mass density may need to account for the possible dependence of κ_{2-10keV} on Eddington ratio.

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arXiv2007

Fields and Filaments in the Core of the Centaurus Cluster

We present high resolution images of the Faraday Rotation Measure (RM) structure of the radio galaxy PKS 1246-410, at the center of the Centaurus cluster. Comparison with Halpha-line and soft X-ray emission reveals a correspondence between the line-emitting gas, the soft X-ray emitting gas, regions with an excess in the RM images, and signs of depolarization. Magnetic field strengths of 25 microG, organized on scales of ~1 kpc, and intermixed with gas at a temperature of 5 x 10^6 K with a density of ~0.1 cm^-3 can reproduce the observed RM excess, the depolarization, and the observed X-ray surface brightness. This hot gas may be in pressure equilibrium with the optical line-emitting gas, but the magnetic field strength of 25 microG associated with the hot gas provides only 10% of the thermal pressure and is therefore insufficient to account for the stability of the line-emitting filaments.

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arXiv2007

A deeper X-ray study of the core of the Perseus galaxy cluster: the power of sound waves and the distribution of metals and cosmic rays

We make a further study of the very deep Chandra observation of the X-ray brightest galaxy cluster, A426 in Perseus. We examine the radial distribution of energy flux inferred by the quasi-concentric ripples in surface brightness, assuming they are due to sound waves, and show that it is a significant fraction of the energy lost by radiative cooling within the inner 75-100 kpc, where the cooling time is 4-5 Gyr, respectively. The wave flux decreases outward with radius, consistent with energy being dissipated. Some newly discovered large ripples beyond 100 kpc, and a possible intact bubble at 170 kpc radius, may indicate a larger level of activity by the nucleus a few 100 Myr ago. The distribution of metals in the intracluster gas peaks at a radius of about 40 kpc and is significantly clumpy on scales of 5 kpc. The temperature distribution of the soft X-ray filaments and the hard X-ray emission component found within the inner 50 kpc are analysed in detail. The pressure due to the nonthermal electrons, responsible for a spectral component interpreted as inverse Compton emission, is high within 40 kpc of the centre and boosts the power in sound waves there; it drops steeply beyond 40 kpc. We find no thermal emission from the radio bubbles; in order for any thermal gas to have a filling factor within the bubbles exceeding 50 per cent, the temperature of that gas has to exceed 50 keV.

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arXiv2007

XMM-Newton observations of the Lockman Hole V: time variability of the brightest AGN

This paper presents the results of a study of X-ray spectral and flux variability on time scales from months to years, of the 123 brightest objects (including 46 type-1 AGN and 28 type-2 AGN) detected with XMM-Newton in the Lockman Hole field. We detected flux variability with a significance >3sigma in ~50% of the objects, including 68+-11% and 48+-15% among our samples of type-1 and type-2 AGN. However we found that the fraction of sources with best quality light curves that exhibit flux variability on the time scales sampled by our data is >80%, i.e the great majority of the AGN population may actually vary in flux on long time scales. The mean relative intrinsic amplitude of flux variability was found to be ~0.15 although with a large dispersion in measured values, from ~0.1 to ~0.65. The flux variability properties of our samples of AGN do not significantly depend on the redshift or X-ray luminosity of the objects and seem to be similar for the two AGN types. Using a broad band X-ray colour we found that the fraction of sources showing spectral variability with a significance >3sigma is ~40% i.e. less common than flux variability. Spectral variability was found to be more common in type-2 AGN than in type-1 AGN with a significance >99%. This result is consistent with the fact that part of the soft emission in type-2 AGN comes from scattered radiation, and this component is expected to be much less variable than the hard component. The observed flux and spectral variability properties of our objects cannot be explained as being produced by variability of one spectral component alone, for example changes in the continuum shape associated with changes in the mass accretion rate, or variability in the amount of X-ray absorption. At least two spectral components must vary in order to explain the X-ray variability of our objects.

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arXiv2007

The luminous X-ray hotspot in 4C 74.26: synchrotron or inverse-Compton emission?

We report the discovery of an X-ray counterpart to the southern radio hotspot of the largest-known radio quasar 4C 74.26 (whose redshift is z=0.104). Both XMM-Newton and Chandra images reveal the same significant (10arcsec, i.e. 19kpc) offset between the X-ray hotspot and the radio hotspot imaged with MERLIN. The peak of the X-ray emission may be due to synchrotron or inverse-Compton emission. If synchrotron emission, the hotspot represents the site of particle acceleration and the offset arises from either the jet exhibiting Scheuer's `dentist's drill' effect or a fast spine having less momentum than the sheath surrounding it, which creates the radio hotspot. If the emission arises from the inverse-Compton process, it must be inverse-Compton scattering of the CMB in a decelerating relativistic flow, implying that the jet is relativistic (Gamma >= 2) out to a distance of at least 800kpc. Our analysis, including optical data from the Liverpool Telescope, rules out a background AGN for the X-ray emission and confirms its nature as a hotspot, making it the most X-ray luminous hotspot yet detected.

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arXiv2007

Ionized nebulae surrounding brightest cluster galaxies

We present IFU observations of six emission-line nebulae that surround the central galaxy of cool core clusters. Qualitatively similar nebulae are observed in cool core clusters even when the dynamics and possibly formation and excitation source are different. Evidence for a nearby secondary galaxy disturbing a nebula, as well as AGN and starburst driven outflows are presented as possible formation mechanisms. One nebula has a rotation velocity of the same amplitude as the underlying molecular reservoir, which implies that the excitation or formation of a nebula does not require any disturbance of the molecular reservoir within the central galaxy. Bulk flows and velocity shears of a few hundred km/s are seen across all nebulae. The majority lack any ordered rotation, their configurations are not stable so the nebulae must be constantly reshaping, dispersing and reforming. The dimmer nebulae are co-spatial with dust features whilst the more luminous are not. Significant variation in the ionization state of the gas is seen in all nebulae through the non-uniform [NII]/H_alpha ratio. There is no correlation between the line ratio and H_alpha surface brightness, but regions with excess blue or UV light have lower line ratios. This implies that UV from massive, young stars act in combination with an underlying heating source that produces the observed low-ionization spectra.

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arXiv2007

A longer XMM-Newton look at I Zwicky 1: Distinct modes of X-ray spectral variability

The short-term spectral variability of the narrow-line Seyfert 1 galaxy I Zwicky 1 (I Zw 1) as observed in an 85 ks XMM-Newton observation is discussed in detail. I Zw 1 shows distinct modes of variability prior to and after a flux dip in the broad-band light curve. Before the dip the variability can be described as arising from changes in shape and normalisation of the spectral components. Only changes in normalisation are manifested after the dip. The change in the mode of behaviour occurs on dynamically short timescales in I Zw 1. The data suggest that the accretion-disc corona in I Zw 1 could have two components that are co-existing. The first, a uniform, physically diffuse plasma responsible for the "typical'' long-term (e.g. years) behaviour; and a second compact, centrally located component causing the rapid flux and spectral changes. This compact component could be the base of a short or aborted jet as sometimes proposed for radio-quiet active galaxies. Modelling of the average and time-resolved rms spectra demonstrate that a blurred Compton-reflection model can describe the spectral variability if we allow for pivoting of the continuum component prior to the dip.

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arXiv2007

A longer XMM-Newton look at I Zwicky 1: physical conditions and variability of the ionised absorbers

We present a spectral analysis of the narrow-line Seyfert 1 galaxy I Zwicky 1, focusing on the characteristics of the ionised absorbers as observed with XMM-Newton in 2005. The soft X-ray spectrum shows absorption by two components of ionised gas with a similar column density (N_H~10^{21} cm^{-2}) and ionisation parameters logξ~0 and 2.5. Comparing this observation with a 2002 XMM-Newton data set, we see a clear anti-correlation between the X-ray ionisation parameter xi_X and the 0.1-10 keV luminosity. Viable explanations for this effect include transient clouds or filaments crossing the line of sight in a complex geometry or a gas observed in non-equilibrium. The outflow velocity of the X-ray low-ionisation absorber is consistent with the outflow of the UV absorber detected in a past Hubble Space Telescope observation. In addition, the ionic column densities of CIV and NV derived from the X-ray model are consistent with the UV values. This suggests that the low-ionisation outflowing gas may survive for many years, despite large changes in flux, and that there is a tight connection between the X-ray and UV absorbers that can only be confirmed with a simultaneous UV and X-ray observation.

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arXiv2007

A longer XMM-Newton look at I Zwicky 1: Variability of the X-ray continuum, absorption, and iron Kalpha line

We present the second XMM-Newton observation (85 ks) of the narrow-line Seyfert 1 galaxy (NLS1) I Zw 1 and describe its mean spectral and timing characteristics. On average, I Zw 1 is ~35 per cent dimmer in 2005 than in the shorter (20 ks) 2002 observation. Between the two epochs the intrinsic absorption column density diminished, but there were also subtle changes in the continuum shape. Considering the blurred ionised reflection model, the long-term changes can be associated with a varying contribution of the power law component relative to the total spectrum. Examination of normalised light curves indicates that the high-energy variations are quite structured and that there are delays, but only in some parts of the light curve. Interestingly, a hard X-ray lag first appears during the most-distinct structure in the mean light curve, a flux dip ~25 ks into the observation. The previously discovered broad, ionised Fe Ka line shows significant variations over the course of the 2005 observation. The amplitude of the variations is 25-45 per cent and they are unlikely due to changes in the Fe ka-producing region, but perhaps arise from orbital motion around the black hole or obscuration in the broad iron line-emitting region. The 2002 data are re-examined for variability of the Fe Ka line at that epoch. There is evidence of energy and flux variations that are associated with a hard X-ray flare that occurred during that observation.

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arXiv2007

Exploring the X-ray spectral variability of MCG-6-30-15 with XMM-Newton

We present a study of the spectral variability of the Seyfert I galaxy MCG-6-30-15 based on the two long XMM-Newton observations from 2000 and 2001. The X-ray spectrum and variability properties of the 2001 data have previously been well described with a two-component model consisting of a variable power law and a much less variable reflection component, containing a broad relativistic iron line from the accretion disc around a rapidly rotating Kerr black hole. The lack of variability of the reflection component has been interpreted as an effect of strong gravitational light bending very close to the central black hole. Using an improved reflection model, we fit the two-component model to time-resolved spectra of both observations. Assuming that the photon index of the power law is constant we reconfirm the old result and show that this does not depend on the time-scale of the analysis.

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